The present invention relates to ink jet printers and, more particularly, to a thermal ink jet printing system feedback from a drop detector to extend print head lifetimes.
Ink jet printers print by propelling ink to selected positions of a print medium, such as paper. The two major classes of ink jet printers are characterized as "drop-on-demand" and "continuous stream" respectively. Drop-on-demand ink jet printers eject ink only when ink is required for printing, whereas continuous stream ink jet printers propel ink in streams and deflect charged drops either to or away from a target medium. A thermal ink jet printer is a drop-on-demand printer which uses heat dissipated in a heater resistor to form and propel ink drops. In the other major type of drop-of-demand printers, e.g. piezo-electric ink jet printers, piezo electric deflection is used to create the pressure necessary to form and propel ink drops.
Although not generally used with thermal ink jet printers, drop detectors have been employed in control subsystems for ink jet printers. Electro-static, piezo-electric and optical drop detectors are known and have been used to determine the presence, speed and position of drops. Some continuous stream ink jet printers use feedback from drop detectors to optimize drop breakoff and charging. U.S. Pat. No. 4,509,057 to Sohl et al. discloses the use of feedback from an optical drop detector to minimize horizontal errors in drop position. Sohl et al. also teach that drop formation is optimized when drop velocity is maintained within a predetermined range. Drop velocity can be calculated from the duration between drop ejection and drop detection. Sohl et al. suggest using this teaching in combination with U.S. Pat. No. 4,459,599 to Donald L. Ort to adjust drive pulses so that drop velocity can be maintained within the velocity range required for optimal drop formation.
Heretofore, drop detectors have not been used to extend the lifetimes of thermal ink jet print heads. Generally, a thermal ink jet print head includes multiple drop generators, which can be used in parallel to increase printing throughput. Typically, each drop generator includes an ink chamber, a heater resistor and an orifice. When an electrical pulse of sufficient energy is applied to the heater resistor, the heat dissipated thereby vaporizes ink in the respective chamber. The volumetric expansion of the ink, resulting from vaporization, forces unevaporated ink through the respective orifice. Contraction of the vapor bubble contributes to breakoff of the ejected ink to form a drop which continues its path to the medium.
Given present day commercial requirements, each heater resistor is expected to deliver at least 40 million drops. Each of these drops corresponds to a rapid heating and cooling of the heater resistor, which is thus subject to considerable thermal fatigue. Thermal fatigue has been shown to aggravate a crack nucleation process, eroding the structural integrity of the heater resistor and its passivation. The effects of thermal fatigue are compounded with mechanical shock during vapor bubble collapse and corrosion from the hot ink liquid and vapor. These compounded effects must be without by a relatively thin heater resistor and its passivation. Failure of a single heater resistor can require replacement of the entire print head. Where the incorporating printer is not designed to use disposable print heads, failure of a single heater resistor means down time, repair costs and/or printer replacement costs.
The importance of limiting thermal fatigue in heater resistors is well recognized. Accordingly, considerable effort has been directed to design of the heater resistor itself, including its compositions and dimensions. In addition, the shape, duration and amplitude of drive pulses have been varied to determine optimal ranges. While some of these efforts have yielded positive results, thermal fatigue remains a limiting factor in thermal ink jet print head lifetimes. To supplement enhancements resulting from optimizing the heater resistor and drive pulse characteristics, as systems approach using feedback could be implemented. However, as explained below, the feedback systems used with continuous stream print heads and with piezo-electric print heads are not directed to minimizing thermal fatigue nor are they obviously adaptable to such a function. What is needed is a feedback system based upon parameters derived from an analysis of thermal ink jet print head operation to minimize thermal fatigue of heater resistors and enhance thermal ink jet print head lifetimes.